The invention relates to electronic hardware used to produce an image of a recurring high speed event with a standard video camera. An imaging method using a standard video camera is described in U.S. Pat. No. 4,970,597 to Shepard. In the Shepard method the camera executes raster patterns wherein the camera repeatedly and sequentially scans a series of parallel horizontal lines which comprise an image field, and generates a video signal comprised of signals representing these lines. The camera continuously views the recurring event and, independently, a flagging signal is generated before each recurrence of the event. Within a complete image field, one or more of the recurring events may begin during an aperture signal that occurs once per line signal. When a recurrence of the event begins during an aperture signal, the line then being scanned will accurately represent a portion of the event. Signals or data representing these accurate or synchronized lines are collected and stored in a computer memory.
The Shepard method uses a frame grabber, which can save and transfer frames or parts of frames to a host computer. The method also uses synchronization hardware to control the frame grabber in response to the flagging signals and signals from the camera. The invention herein is a design for this synchronization hardware. It should be noted that the Shepard patent describes the imaging method for which the synchronization hardware disclosed herein is designed.
The Shepard method uses a commercially available infrared imaging camera having RS-170 video output. The raster pattern of the camera is traced by two sinusoidally driven scanning mirrors, one driven vertically and one driven horizontally. The vertical scanning mirror is driven at a frequency of 60 Hz and the horizontal scanning mirror is driven at a frequency of 4 kHz. The vertical mirror frequency is appropriate for generating a video signal, but the horizontal mirror frequency is one fourth as fast as is required for RS-170 video output.
The camera has two features to remedy the lack of sufficient speed of the horizontal scanning mirror. First, the camera accepts input from the horizontal scanning mirror during both the forward and reverse passes. The camera digitizes the data it receives from the horizontal mirror on the reverse passes. Since the camera receives this data in reverse order, the camera again reverses the data to put this data in proper, "forward scan" order. The digitizing of information on the reverse passes increases the effective frequency of the horizontal mirror but also decreases the accuracy of this information when rapidly changing events are viewed.
The second feature increasing the effective frequency of the horizontal scanning mirror is the output of each scanned line in duplicate. Thus, if the first horizontal line of the camera's raster pattern output is a forward scanned horizontal line, then the second line of the camera's raster pattern output is a copy of the first line. The third line of the camera's raster pattern output will be a reverse scanned output and the fourth line will be a copy of the reverse scanned third line. The raster pattern output from the camera will be repetitions of the sequence of the four lines just described.
It may be desirable in some cases to utilize only data from all forward scanned lines, or only from all reverse scanned lines, or perhaps from both forward and reverse scan lines of the camera raster pattern. To accomplish this, signals generated by the motion of the horizontal scanning mirror are monitored. The synchronization hardware described herein is capable of receiving these signals. The synchronization hardware has circuitry to exert part of its control over the frame grabber in response to these signals. The synchronization hardware can be manually adjusted so that frame grabber accepts only forward scan lines, both forward and reverse scan lines, or only reverse scan lines.